1. Technical Field
The invention relates generally to integrated circuit (IC) chip fabrication, and more particularly, to methods of improving operational parameters of at least one pair of matched transistors, and a set of transistors.
2. Background Art
In the integrated circuit (IC) chip fabrication industry, designers often make transistors physically larger to improve threshold voltage (Vt) matching characteristics between transistors. Typically, the matching problem is not a geometric problem, but depends on the amount of dopants present in respective channel regions under gates of the transistors, which may vary from transistor to transistor. One technique to ensure threshold voltage matching is to assume that the threshold voltage mismatch is proportional to the square root of area of each of the transistors, assuming that the transistors are intended to be identical. One common way to match threshold voltages between paired transistors is to lengthen a channel of one or more of the transistors to adjust the area of the channel of the transistor. Modern transistors, however, have increasingly small channels and non-uniform lateral doping profiles. For these transistors, the degree of threshold voltage mismatch is not always proportional to the area. That is, with some transistors, e.g., those with a heavy halo (or pocket) implant, the threshold voltage (Vt) mismatch will not improve with channel lengthening as rapidly as predicted by the simple square root of area theory.
FIG. 1 shows a set 90 of matched transistors 100A, 100B that are matched according to embodiments of a conventional method of reducing a threshold voltage (Vt) mismatch between transistors for analog applications, i.e., increased channel length relative to the minimum otherwise required by the technology limitations. As illustrated, transistor 100A includes a conventional transistor including a channel 110 (under gate 116) having contacts 112 to source/drain regions 114. Transistors 100A, 100B each include a single gate 116, e.g., of a polysilicon, extending over channel 110. A threshold voltage (Vt) of transistor 100A is to be substantially matched to that of transistor 100B. Illustrative data is shown in FIG. 2 from a 65 nm low-power CMOS technology. The apparent mismatch slope (i.e., mismatch per square root of device area) increases monotonically at longer channel lengths.
In view of the foregoing, there is a need in the art for another mechanism of improving operational parameters such as threshold voltage mismatch for a set of matched transistors.